I. Field of the Invention
This invention relates to computerized tomography and more particularly to compression and recovery of medical imaging data.
II. Description of the Related Art
Medical imaging requires the generation, processing, and storage of prodigious quantities of digital image data. Digital image compression is employed to represent an image by using as few bits as possible consistent with a high degree of image fidelity, with the goals of reducing data processing and transmission time and reducing data storage space requirements.
Compression algorithms attempt to produce a representation of a given image in terms of uncorrelated data samples. Data representing a spatial characteristic of the image, such as image intensity as a function of position, are highly correlated, that is, given information about some data values, other data values can be interpolated. There is, therefore, redundant information in spatial image data which prevents maximum data compression for a given image quality.
Digital image compression balances the degree of compression against the degradation of the reconstructed image, i.e., the image generated from the compressed and then decompressed data. Transform compression of image data is one method of obtaining high compression ratios with only modest image degradation. The compression ratio is the ratio of the number of data bits needed to represent the uncompressed image divided by the number of data bits needed to represent the same image in a compressed format. Typical of such ratios would be 20 bits compressed to 1 bit, written as 20:1.
Compression is also described in terms of bits per pixel. Typically, 10 data bits represent one medical image pixel. Lossless compression techniques are capable of achieving a reduction to about 4.0 bits per pixel. However it is desirable to employ transform coding procedures to produce images compressed at a coding rate of 0.5 bits per pixel. At this lower limit image smoothing results from the loss of high frequency information. Also, compression methods that break images into many subimages, or blocks, may accentuate subimage discontinuities at the subimage boundaries. Finally, procedures which perform compression by selecting only the most dominant topographic features within image blocks may randomly choose some subset of components representative of the noise content. Upon decompression, this partial information can result in the appearance of subjectively objectionable spurious medium-to-high frequency two dimensional patterns.
Typical solutions to these image degradation problems involve reduction in the compression ratio to retain more of the image details or application of image processing techniques to restore degraded images. Each of these solutions detracts from the stated goals of data compression by increasing data storage requirements, increasing processing times, increasing data transmission times, or all of the three. In some cases, such as in computation methods to improve the appearance of block edge boundaries, intrablock artifacts may be introduced. Thus the solution to a first problem results in the introduction of a second problem.
It is therefore an object of the present invention to provide a method and apparatus to substantially reduce visual artifacts in reconstructed medical image data without substantially increasing data storage or processing time requirements.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.